A recently committed revison of R-devel (47740) has introduced a new
mechanism for ordering superclasses consistently, with related changes
for selecting inherited methods.

As part of the process, a function testInheritedMethods has been
introduced that examines method selection for the relevant subclasses
and reports ambiguities.

Maintainers of packages that have methods involving multiple arguments
are encouraged to run testInheritedMethods for the relevant generic
functions (e.g., the binary operators). The new method selection is
unambiguous for single-argument selection.

It's preferable to find such ambiguities during package development or
revision, rather than having users encounter ambiguous method selection
later on. In that spirit, ambiguous method selection is no longer a
warning, just a message.